The present disclosure relates generally to methods for forming electrolyte compositions, and more particularly to the synthesis of an electrolyte solution for use in ultracapacitors.
Energy storage devices such as ultracapacitors may be used in many applications where a discrete power pulse is required. Such applications range from cell phones to hybrid vehicles. An important characteristic of an ultracapacitor is the energy density that it can provide. The energy density of the device, which can comprise two or more carbon-based electrodes separated by a porous separator and/or an organic electrolyte, is largely determined by the properties of the electrolyte. A typical electrolyte utilized in commercial ultracapacitors comprises tetraethyl ammonium tetrafluoroborate (TEA-TFB) salt dissolved in a solvent such as acetonitrile. This electrolyte system has a number of beneficial properties, including salt solubility and ion conductivity.
One factor that is important in the development of electrolyte solutions is cost. Due to its relatively expensive synthesis and purification, commercially-available TEA-TFB is expensive. An example synthesis of TEA-TFB is disclosed in U.S. Pat. No. 5,705,696. The example process involves reacting tetraalkyl ammonium halides with metal tetrafluoroborates in an aqueous medium followed by membrane dialysis to remove metal halides. Another synthesis approach is disclosed in U.S. Pat. No. 7,641,807, which discloses combining a metal halide and a tetraalkyl halide in acetonitrile followed by filtering of the metal halide. The product of this process typically includes a high concentration of halide ions, such as chloride ions (e.g., 0.71 wt. % or 7100 ppm) as well as associated metal ions. Such a concentration of halide ions is understood to be detrimental to ultracapacitor performance.
In view of the foregoing, there is a need for a simple and economical synthesis process to produce high purity TEA-TFB salt and electrolyte solutions comprising TEA-TFB salt.